This invention relates to a process for the production of aromatic carboxylic acids such as terephthalic acid, isophthalic acid, trimellitic acid, naphthalene dicarboxylic acid and benzoic acid.
Terephthalic acid, by way of an example, is an important intermediate for the production of polyester polymers which are used typically for fibre production and in the manufacture of bottles. Current state-of-the-art technology for the manufacture of terephthalic acid involves the liquid phase oxidation of paraxylene feedstock using molecular oxygen in a lower (e.g. C2-C6) aliphatic monocarboxylic acid, usually acetic acid, in the presence of a dissolved heavy metal catalyst system usually incorporating a promoter, such as bromine. Acetic acid is particularly useful as the solvent since it is relatively resistant to oxidation and increases the activity of the catalytic pathway. The reaction is carried out in a stirred vessel under elevated temperature and pressure conditions, typically 150 to 250xc2x0 C. and 6 to 30 bara, respectively, and typically produces terephthalic acid in high yield, e.g. at least 95%.
Generally, however, the terephthalic acid obtained is not sufficiently pure for direct use in polyester production since it contains, as major impurities, partially-oxidised intermediates of terephthalic acid, particularly 4-carboxybenzaldehyde (4-CBA), along with various color-forming precursors and colored impurities. In a conventional process used for the production of terephthalic acid, a substantial proportion of the terephthalic acid tends to precipitate as it forms during the course of the reaction and, although it may be below its solubility limit in the solvent under the prevailing conditions, 4-CBA tends to co-precipitate with the terephthalic acid. This relatively crude terephthalic acid, therefore, has to be processed further to secure terephthalic acid of acceptable quality for use in production of high grade polyester. Such further processing typically comprises dissolving the impure terephthalic acid in water at an elevated temperature to produce a solution which is hydrogenated in the presence of a suitable catalyst, e.g. a noble metal catalyst on a carbon support. This hydrogenation step converts the 4-CBA to para-toluic acid while the various color bodies present in the relatively impure terephthalic acid are converted to colourless products. The purified terephthalic acid is then recovered from solution by a series of crystallisation, solid-liquid separation and drying steps. Because para-toluic acid is considerably more soluble in water than terephthalic acid, the former tends to remain in the aqueous mother liquor following crystallisation and solids-liquid separation. A process involving production of crude terephthalic acid and its subsequent purification by hydrogenation is disclosed in, for example, EP-A-0498591 and EP-A-0502628.
In a continuous process described in WO-A-98/38150, relatively high solvent/precursor ratios are employed, and, accordingly, substantially all of the aromatic carboxylic acid produced can be kept in solution thereby minimising co-precipitation of the reaction intermediates in the course of the reaction. As a result, the intermediates remain available for reaction to the desired aromatic carboxylic acid, and the rate of reaction is enhanced for the intermediates compared with a conventional process. By operating the oxidation reaction in this way, it is possible to reduce the extent of contamination of the aromatic carboxylic acid with any aldehyde produced as an intermediate in the course of the reaction. For instance, as mentioned above, in the case of terephthalic acid production by liquid phase oxidation of paraxylene or other precursor, the reaction results in the production of 4-carboxybenzaldehyde as an intermediate. Co-precipitation of 4-CBA with terephthalic acid is largely avoided since the terephthalic acid is not allowed to precipitate during the reaction, at least not to any substantial extent. Moreover, the conditions necessary to achieve this tend to lead to oxidation of intermediates such as 4-CBA to a greater extent to the desired end product.
Although, the process described in WO-A-98/38150 represents a valuable improvement over the prior art, it involves the use of substantial amounts of organic solvent. Although organic solvents, such as acetic acid, are particularly useful in such oxidation processes for the reasons given above, it would in certain situations be desirable to minimise their use. Such organic solvents are relatively costly and, due to environmental restrictions, may require recovery and recycling Furthermore, a proportion of the organic solvent may be xe2x80x98lostxe2x80x99 due to combustion during the oxidation reaction. A further problem with the use of acetic acid is that it is flammable when mixed with air or oxygen under typical reaction conditions in this system.
A further problem with the use of conventional solvents, such as acetic acid, is the low solubility of the oxidant component therein. Thus, where dioxygen is used as the oxidant, the dioxygen is present predominantly as discrete bubbles in the reaction medium with only a small proportion of the dioxygen dissolving in the solvent. To the extent that the reaction between the precursor and the dioxygen results from the dioxygen diffusing from the bubbles into the bulk liquid, the reaction rate is limited by the low solubility of dioxygen in the solvent.
Holliday R. L. et al (J. Supercritical Fluids 12, 1998, 255-260) describe a batch process for the synthesis of, inter alia, aromatic carboxylic acids from alkyl aromatics in a reaction medium of sub-critical water using molecular oxygen as the oxidant. The dielectric constant of water decreases dramatically from a room temperature value of around 80 C2/NM2 to a value of 5 C2/NM2 as it approaches its critical point (374xc2x0 C. and 220.9 bara), allowing it to solubilise organic molecules. As a consequence, water then behaves like an organic solvent to the extent that hydrocarbons, e.g. toluene, are completely miscible with the water under supercritical conditions or near supercritical conditions. Dioxygen is also highly soluble in sub- and super-critical water. The process described by Holliday et al was carried out in sealed autoclaves as a batch reaction.
It is an object of this invention to provide an alternative and improved continuous process for the production of an aromatic carboxylic acid, such as terephthalic acid, wherein substantially all of the aromatic carboxylic acid produced, i.e., intermediates and precursors, are maintained in solution during the reaction, and wherein the need to use an organic material, such as aliphatic monocarboxylic acid, as solvent is eliminated. It is a further object of this invention to provide an alternative and improved continuous process for the production of an aromatic carboxylic acid wherein substantially all of the reactants and product are maintained in a common phase during reaction. It is a further object of this invention to provide a continuous process, having good selectivity and high yield, for the production of an aromatic carboxylic acid by the oxidation of a precursor in sub- or super-critical water.
We have now devised a process which overcomes one or more of the problems previously encountered for the use of supercritical water.
According to the present invention there is provided a process for the production of an aromatic carboxylic acid comprising contacting in the presence of a catalyst, within a continuous flow reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions close to the supercritical point such that said one or more precursors, oxidant and aqueous solvent constitute a substantially single homogeneous phase in the reaction zone, wherein the contact of at least part of said precursor with said oxidant is contemporaneous with contact of said catalyst with at least part of said oxidant. Substantially all the aromatic carboxylic acid produced is maintained in solution during the reaction, and thereafter the aromatic carboxylic acid is recovered from the reaction medium.
By employing water under supercritical or near supercritical conditions, the desired aromatic carboxylic acid can be produced without employing aliphatic carboxylic acids, such as acetic acid, as the primary solvent.
The process is carried out with the reactants and the solvent forming a substantially single homogeneous fluid phase in which the components in question are mixed at a molecular level. This is in contrast with existing processes where the dioxygen is present as discrete bubbles in the reaction medium, e.g. acetic acid. To the extent that the reaction between the precursor, e.g. paraxylene, and dioxygen results from dioxygen diffusing from the bubbles into the bulk liquid, the reaction rate of the known process is limited by the solubility of dioxygen in acetic acid, which is not high. The use of water under supercritical or near supercritical conditions as the solvent operates to transform the reaction kinetics, since the concentration of dioxygen in water increases markedly as the supercritical point is approached and exceeded. Moreover, the reaction kinetics are further enhanced by the high temperatures prevailing when the water solvent is under supercritical or near supercritical conditions. The combination of high temperature, high concentration and homogeneity mean that the reaction to convert the precursor(s) to aromatic carboxylic acid can take place extremely rapidly compared with the residence times employed in the production of aromatic carboxylic acids, such as terephthalic acid, by conventional techniques using a crystallising three phase oxidation reactor. Under the conditions described herein according to the invention, the intermediate aldehyde (e.g. 4-CBA in the case of terephthalic acid) can be readily oxidised to the desired aromatic carboxylic acid which is soluble in the supercritical or near supercritical fluid thereby allowing a significant reduction in contamination of the recovered aromatic carboxylic acid product with the aldehyde intermediate. As noted above, in the conventional prior art process of oxidising paraxylene to terephthalic acid, the terephthalic acid is only sparingly soluble in the aliphatic carboxylic acid solvent, and it precipitates in the course of the reaction; because the conversion of 4-CBA to terephthalic acid proceeds relatively slowly, 4-CBA, therefore, tends to co-precipitate with the terephthalic acid, both during the reaction and during the subsequent recovery of the terephthalic acid.
The process of the present invention is particularly advantageous in that it substantially overcomes the problems of autocatalytic destructive oxidation of the precursor and consumption of the catalyst. Furthermore, the process of the present invention involves short residence times and exhibits high yield and good selectivity of product formation.
In the process of the invention, the pressure and temperature of the process are selected to secure supercritical or near supercritical conditions. Thus, operating temperatures are typically in the range of from 300xc2x0 to 480xc2x0 C., more preferably 330xc2x0 to 450xc2x0 C., typically from a lower limit of about 350xc2x0 to 370xc2x0 C. to an upper limit of about 370xc2x0 to about 420xc2x0 C. Operating pressures are typically in the range from about 40 to 350 bara, preferably 60 to 300 bara, more preferably 220 to 280 bara, and particularly 250 to 270 bara.
By xe2x80x9cnear supercritical conditionsxe2x80x9d we mean that the reactants and the solvent constitute a substantially single homogeneous phase; in practice, this can be achieved under conditions below the critical temperature for water. According one embodiment, the term xe2x80x9cnear supercritical conditionsxe2x80x9d means that the solvent is at a temperature which is not less than 50xc2x0 C. below, preferably not less than 35xc2x0 C. below, more preferably not less than 20xc2x0 C. below the critical temperature of water at 220.9 bara.
By xe2x80x9ccontinuous flow reactorxe2x80x9d as used herein we mean a reactor in which reactants are introduced and mixed and products withdrawn simultaneously in a continuous manner, as opposed to a batch-type reactor. For example, the reactor may be a plug flow reactor, although the various aspects of the invention defined herein are not limited to this particular type of continuous flow reactor.
In the process of the invention, substantially all, and in any event no less than 98% by wt, of the aromatic carboxylic acid produced in the reaction is maintained in solution during the reaction and does not begin to precipitate until the solution leaves the oxidation reaction zone and undergoes cooling.
By carrying out the process in a continuous flow reactor, the residence time for the reaction can be made compatible with the attainment of conversion of the precursor(s) to the desired aromatic carboxylic acid without significant production of degradation products. The residence time of the reaction medium within the reaction zone is generally no more than 10 minutes. However, in practice the reaction runs to completion almost instantaneously as the reactants are mixed, and, therefore, the xe2x80x9cresidence timexe2x80x9d of the reactants in the reaction zone is very short, usually on the order of 2 minutes or less.
The residence time may be controlled so that the precursor is converted rapidly to the corresponding aromatic carboxylic acid with such high efficiency that the aromatic carboxylic acid precipitated from the reaction medium following completion of the reaction contains substantially low levels of aldehyde intermediate, e.g., no more than about 5000 ppm, but even as low as 1500 ppm, and in some cases no more than about 500 ppm aldehyde produced as an intermediate in the course of the reaction (e.g. 4-CBA in the case of terephthalic acid production). Typically, there will be at least some aldehyde present after the reaction, and usually at least 5 ppm.
The reactor system suitable for performing the process of the present invention may be generally configured as described below.
There may be more than one reaction zone in series or in parallel. For instance, where multiple reaction zones in parallel are used, the reactants and solvent may form separate flow streams for passage through the reaction zones and, if desired, the product streams from such multiple reaction zones may be united to form a single product stream. Where more than one reaction zone is used, the conditions, such as temperature, may be the same or different in each reactor. Each reactor may be operated adiabatically or isothermally. Isothermal or a controlled temperature rise may be maintained by heat exchange to define a predetermined temperature profile as the reaction proceeds through the reactor.
In one embodiment of the invention, the heat of reaction is removed from the reaction by heat exchange with a heat-accepting fluid, according to conventional techniques known to those skilled in the art.
In one embodiment, the heat-accepting fluid is passed through one or more flow passages having a wall or walls, the outer surfaces of which are exposed to the reaction medium within the reaction zone. For example, the reactor may be designed in a manner akin to a tube and shell heat exchanger with the reactants and solvent being passed through the shell and the heat-accepting fluid being passed through the tubes internally of the shell.
However, we do not exclude the possibility of effecting the thermal transfer in other ways, for instance by passing the heat-accepting fluid through a jacket arrangement at least partly surrounding the reaction zone. For example, the tube in shell design referred to above may be such that the reactants and solvent flow through the tubes while the heat-accepting fluid flows through the shell.
The heat-accepting fluid may traverse the reaction zone in countercurrent and/or co-current relation with the reaction medium flowing through the reaction zone. Conveniently the passage or passages conducting the heat-accepting fluid are arranged to extend internally of the reactor.
Advantageously, the heat-accepting fluid following heat exchange with the reaction medium is processed to recover thermal, mechanical and/or electrical energy. The power recovered may in part be employed to pressurise air or oxygen to be supplied as oxidant to the process, e.g. by driving a compressor suitable for this purpose. For example, heat transferred to the heat-accepting fluid may be converted to mechanical or electrical energy in a power recovery system. One approach is to use the heat-accepting fluid to raise high pressure steam which can then be superheated and supplied to a steam turbine to recover power. Sufficient power may be recovered to allow export from the plant for use elsewhere.
Conveniently the heat-accepting fluid comprises water.
The heat-accepting fluid may be preheated prior to traversing the reaction zone, and such preheating may be effected by heat exchange with the product stream resulting from the oxidation reaction.
The oxidant in the process of the invention is preferably molecular oxygen, e.g. air or oxygen enriched air, but preferably comprises gas containing oxygen as the major constituent thereof, more preferably pure oxygen, or oxygen dissolved in liquid. The use of air is not favoured, although not excluded from the scope of the invention, since large compression costs would arise and offgas handling equipment would need to cope with large amounts of offgas owing to the high nitrogen content of air. Pure oxygen or oxygen enriched gas on the other hand permits use of a smaller compressor and smaller offgas treatment equipment. The use of dioxygen as the oxidant in the process of the invention is particularly advantageous since it is highly soluble in water under supercritical or near supercritical conditions. Thus, at a certain point, the oxygen/water system will become a single homogeneous phase.
Instead of molecular oxygen, the oxidant may comprise atomic oxygen derived from a compound, e.g. a liquid phase compound at room temperature, containing one or more oxygen atoms per molecule. One such compound for example is hydrogen peroxide, which acts as a source of oxygen by reaction or decomposition as described by Lin, Smith, et al (International Journal of Chemical Kinetics, Vol 23, 1991, p971).
The process of the invention is carried out in the presence of an oxidation catalyst. The catalyst may be soluble in the reaction medium comprising solvent and the aromatic carboxylic acid precursor(s) or, alternatively, a heterogeneous catalyst may be used. The catalyst, whether homogeneous or heterogeneous, typically comprises one or more heavy metal compounds, e.g. cobalt and/or manganese compounds, and may optionally include an oxidation promoter. For instance, the catalyst may take any of the forms that have been used in the liquid phase oxidation of aromatic carboxylic acid precursors such as terephthalic acid precursor(s) in aliphatic carboxylic acid solvent, e.g. bromides, bromoalkanoates or alkanoates (usually C1-C4 alkanoates such as acetates) of cobalt and/or manganese. Compounds of other heavy metals, such as vanadium, chromium, iron, molybdenum, a lanthanide such as cerium, zirconium, hafnium, and/or nickel may be used instead of cobalt and/or manganese. Advantageously, the catalyst system will include manganese bromide (MnBr2). The oxidation catalyst may alternatively or additionally include one or more noble metals or compounds thereof, e.g. platinum and/or palladium or compounds thereof, for example in highly divided form or in the form of a metal sponge. The oxidation promoter where employed may be in the form of elemental bromine, ionic bromide (e.g. HBr, NaBr, KBr, NH4Br) and/or organic bromide (e.g. bromobenzenes, benzyl-bromide, mono- and di-bromoacetic acid, bromoacetyl bromide, tetrabromoethane, ethylene-di-bromide, etc.). Alternatively the oxidation promoter may comprise a ketone, such as methylethyl ketone, or aldehyde, such as acetaldehyde.
Where the catalyst is in heterogeneous form, it may be suitably located within the reaction zone so as to secure contact between the continuously flowing reaction medium and the catalyst. In this event, the catalyst may be suitably supported and/or constrained within the reaction zone to secure such contact without unduly constricting the flow cross-section. For instance, the heterogeneous catalyst may be coated on or otherwise applied to, or embodied in, static elements (e.g. elements forming an openwork structure) positioned of within the reaction zone so that the reaction medium flows over the same. Such static elements may additionally serve to enhance mixing of the reactants as they pass through the reaction zone. Alternatively the catalyst may be in the form of mobile pellets, particles, finely divided form, metal sponge form or the like with means being provided if necessary to confine the same to the reaction zone so that, in operation, the catalyst pellets etc become suspended or immersed in the reaction medium flowing through the reaction zone. The use of a heterogeneous catalyst in any of these ways confers the advantage of being able to confine the catalysis effect to a well-defined zone so that, once the reaction medium has traversed the zone, further oxidation takes place at a reduced rate or may be significantly suppressed.
The support for the oxidation catalyst can be less catalytically active or even inert to the oxidation reaction. The support may be porous and typically has a surface area, including the area of the pores on the surface, of at least 25 m2/gm to 250 m2/gm, e.g. from 50 m2/gm to 200 m2/gm, with a surface area of about 80 m2/gm to about 150 m2/gm being preferred. The catalyst support materials should be substantially corrosion resistant and substantially oxidation resistant under the conditions prevailing. The support component of the oxidation catalyst may be pure or a composite of materials, the latter being employed for example to impart desired chemical or physical characteristics to the catalyst. In a preferred embodiment, the catalyst support material comprises zirconium dioxide.
The oxidation reaction is initiated by heating and pressurising the reactants followed by bringing the heated and pressurised reactants together in a reaction zone. This may be effected in a number of ways with one or both of the reactants being admixed with the aqueous solvent prior to or after attainment of supercritical or near supercritical conditions, such admixture being effected in such a way as to maintain the reactants isolated from one another until brought together in the reaction zone.
In the continuous process of the present invention, the reactor system is configured such that the contact between the oxidant and at least part, and preferably substantially all, of the precursor is made at the same point in the reactor system as the contact between the catalyst and at least part, and preferably substantially all, of the oxidant.
In a first embodiment, the oxidant is mixed with the aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state, with suitable pressurisation and, if desired, heating, of the oxidant prior to mixing with the aqueous solvent. The precursor is subjected to pressurisation and, if desired, heating. In the case of a process using a homogeneous catalyst, the catalyst component is subjected to pressurisation and, if desired, heating. The precursor, the catalyst and the oxidant/solvent mixture are then contacted simultaneously. In the case of a process using a heterogeneous catalyst, the precursor is contacted with the oxidant/solvent mixture in the presence of the catalyst.
In a second embodiment of the invention, the precursor is mixed with the aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state, with suitable pressurisation and, if desired, heating, of the precursor prior to mixing with the aqueous solvent. In one arrangement, a homogenous catalyst component, after pressurisation and optional heating, is contacted with the aqueous solvent simultaneously with the contacting of the precursor with the aqueous solvent. In an alternative arrangement, a heterogeneous catalyst is used and confined to the reaction zone as described herein. The oxidant after pressurisation and, if desired, heating, is mixed with aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state. In the case of a process using a homogeneous catalyst, the oxidant/aqueous solvent mixture is then contacted with the mixture comprising the precursor, catalyst and aqueous solvent. In the case of a process using a heterogeneous catalyst, the oxidant/aqueous solvent mixture is contacted in the reaction zone, i.e. in the presence of the heterogeneous catalyst, with the mixture comprising the precursor and aqueous solvent.
Contact of the various streams may be effected by way of separate feeds to a device in which the feeds are united to form a single homogeneous fluid phase thus allowing the oxidant and precursor to react. The device in which the feeds are united may for instance have a Y, T, X or other configuration allowing separate feeds to be united in a single flow passage forming the continuous flow reactor, or in some circumstances multiple flow passages forming two or more continuous flow reactors. The flow passage or passages in which the feeds are united may comprise a section of tubular configuration with or without internal dynamic or static mixing elements.
In a preferred embodiment, in-line or static mixers are advantageously used to ensure rapid mixing and homogeneity, for example to promote dissolution of oxidant into the aqueous solvent and the formation of a single phase.
The oxidant feed and the precursor feed may be brought together at a single location or the contact may be effected in two or more stages so that at least part of one feed or of both feeds are introduced in a progressive manner, e.g. via multiple injection points, relative to the direction of flow through the reactor. For instance, one feed may be passed along a continuous flow passage into which the other feed is introduced at multiple points spaced apart lengthwise of the continuous flow passage so that the reaction is carried out progressively. The feed passed along the continuous flow passage may include the aqueous solvent as may the feed introduced at multiple positions.
Similarly, the addition of catalyst, particularly homogenous catalyst, may be effected in a progressive manner, e.g. via multiple injection points, relative to the direction of flow through the reactor.
In one embodiment, the oxidant is introduced to the reaction at two or more locations. Such locations are conveniently so positioned relative to the bulk flow of solvent and reactants through the oxidation zone that oxidant is introduced to the reaction at an initial location and at least one further location downstream of said initial location.
After traversing the continuous flow reactor, the reaction mixture comprises a solution of aromatic carboxylic acid. In contrast with conventional prior art processes, substantially the entire amount of aromatic carboxylic acid produced in the reaction is in solution at this stage. The solution may also contain catalyst (if used), and relatively small quantities of by-products such as intermediates (e.g. p-toluic acid and 4-CBA in the case of terephthalic acid), decarboxylation products such as benzoic acid and degradation products such as trimellitic anhydride and any excess reactants. The desired product, aromatic carboxylic acid, such as terephthalic acid, may be recovered by causing or allowing the aromatic carboxylic acid to crystallise from the solution in one or more stages followed by a solids-liquid separation in one or more stages.
Another aspect of the invention is concerned with cooling of the product stream resulting from the oxidation reaction. In this aspect of the invention, the product stream is subjected to a solids-liquid separation to recover the aromatic carboxylic acid and the mother liquor (which may but need not necessarily contain dissolved catalyst components) is recycled to the oxidation reaction zone.
Preferably prior to re-introduction into the oxidation reaction zone, the mother liquor is heated by heat exchange with the product stream thereby cooling the latter.
One or both reactants may be admixed with the mother liquor recycle stream or separate mother liquor recycle streams prior to re-introduction of the mother liquor into the reaction zone and the mother liquor recycle stream (or at least that fraction or those fractions thereof to be combined with the reactant or reactants) may be heated and pressurised to secure supercritical/near supercritical conditions before being admixed with the reactant or respective reactant.
Where the mother liquor is heated by heat exchange with the product stream before re-introduction into the oxidation zone, the reactant or reactants may be admixed with the mother liquor stream or a respective mother liquor stream prior to or after such heat exchange with the product stream.